1. Field of the Invention
The present invention relates to a lens sheet such as a Fresnel lens or lenticular lens used for a screen of a video projection television, or a condensing Fresnel lens.
2. Description of the Related Art
In the production of a lens sheet such as a Fresnel lens or lenticular lens, when the size is relatively small and mass production is carried out, an injection molding process for synthetic resins is generally adopted. When a relatively large lens, such as the above-mentioned screening lens or condensing lens, is prepared, a process is generally adopted in which a resin plate is fitted to a flat lens mold and is heated and pressed to transfer the lens surface profile to the resin plate. In the latter process, however, a long time is needed for heating and cooling, and the process is disadvantageous in that the productivity cannot be increased.
As the means for eliminating this disadvantage, there has been proposed a process comprising interposing an ultraviolet-curable resin liquid between a lens mold and a transparent substrate and curing the resin under irradiation with ultraviolet rays (see Japanese Unexamined Patent Publication No. 61-177215), and a transmission type screen comprising a Fresnel lens or lenticular lens prepared by this process is known (see Japanese Unexamined Patent Publication No. 63-167301).
When a lens sheet as mentioned above is prepared, an ultraviolet-curable resin used for this production must have such properties as adhesion to a transparent substrate, release from a lens mold, strength and shape-retaining of a lens portion after curing, and light transmission.
The processes disclosed in the above-mentioned two patent publications will now be discussed with reference to these properties. In the former process, ultraviolet-curable resins are not specifically explained, and in the screen of the latter technique, it is taught that an ultraviolet-curable resin having a viscosity not larger than 500 cps before curing, a polymerization shrinkage not larger than 10% at curing, 10 and a modulus of elasticity of 5,000 to 30,000 kg/cm.sup.2 after curing is used.
If this ultraviolet-curable resin is used, however, the Young's modulus is too high and removal from the lens mold is difficult, and if the molded lens is forcibly removed from the mold, the lens portion is sometimes chipped. Moreover, the specific ultraviolet-curable resin most preferably used in this technique is discolored after the curing, and thus it is obvious that this resin is not suitable for a lens.